ata_piix: remove SIDPR locking

Now that libata provides proper cross-port EH exclusion.  The SIDPR
locking added by commit 213373cf (ata_piix: fix locking around SIDPR
access) is no longer necessary.  Remove it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>